A corn steeping process as well as a process and apparatus for processing corn.
This invention relates to a corn steeping process wherein corn grains in one or more vessels are contacted with sulphur dioxide containing steeping liquid at a temperature of from 40.degree. to 55.degree. C. In the manufacture of corn starch, it is customary to subject fresh purified corn grains to a prolonged steeping treatment in an aqueous solution of sulphur dioxide and/or lactic acid in order to prepare these grains for subsequent milling and starch separating operations. During such steeping treatment, the aqueous solution extracts soluble substances from the corn kernels. The resulting steep liquor may be used as a nutrient medium for microbiological fermentations or in concentrated form as a component of cattle feed.
The steeping treatment is usually effected in vertical reaction vessels by filling these vessels with a batch of corn, adding an amount of steeping liquor and applying steam to maintain the temperature at about 50.degree. C. After completion of the treatment the liquor is drained off and the corn is removed from the vessel and fed to a milling station.
In practice, a number of reaction vessels, e.g. six to ten or more are positioned next to each other and are filled one after the other with a batch or corn. In this way, the corn in the first filled vessel has undergone already a relatively long steeping treatment at the time of filling the last. The steeping liquor is passed then through all these vessels in series, first entering the vessel in which the corn has already undergone a long steeping treatment, them passing to a vessel in which the corn has undergone a shorter steeping treatment etc., and finally entering the vessel filled with fresh corn.
In each of these vessels, the steeping liquor will extract soluble substances from the corn so that it will grow gradually more concentrated in soluble components.
Thus, whereas the corn wet milling process following the steeping process is a continuous process, the steeping is static, semicontinuous and therefore time consuming and inefficient. The steeping time varies from 32 to 60 hours depending upon the corn quality.
In the current conventional steeping-system the liquid flow is from top to bottom. The corn sits in the vessels and each kernel is more than 65% covered by other corn kernels. All corn grains remain stationary during the whole steeping treatment which will increase the irregularity of the liquor flow around the corn kernels. During the steeping treatment the kernels swell by absorption of water. However, in a large steep vessel corn does not appear to increase in volume because the weight of grain forces the swelling kernels to reorient to fill the irregular voids between the kernels. When said voids become smaller, the velocity of the liquid around the kernels decreases even more and so does the mass transfer. Therefore, the efficiency of steeping treatment decreases in time.
Many efforts have been carried out to reduce the steeping time. Kempf, Die Starke, 23 (1971) Nr. 3, pages 89-95, mentions that in laboratory tests it has been found that using a mechanical movement of corn in comparison with the conventional corn steeping process equal starch yields may be obtained at a shortened steeping time of about 30 hours maintaining equal quality properties of the obtained starch. However, the quantitative and qualitative results are only comparable with those obtained after conventional steeping at marginally reduced steeping times.
Hassanean, Die Starke, 38 (1968) Nr. 12, pages 417-419, steeped corn grains by using two methods in laboratory. The first method was the usual counter-current system for 50 hours as a control. The second method was the counter-current system with agitation at low speed (150 r.p.m.) for 10 hours. The steeping agent in the first 5 hours was a used sulphurous acid solution and in the second 5 hours a fresh SO.sub.2 solution.
The results show that the moisture and protein contents of intact grains after both steeping treatments are equal and also the starch yields are quite similar. However, changing the steeping liquor will cause problems when applied on industrial scale. Further, in this article it is reported that in a plant the steeping time could be reduced to 25 hours by using the counter-current system with circulation of corn grains and corn steepwater.
The agitation and mechanical movement proposed in the above prior art are highly energy and maintenance intensive and difficult to carry out on full scale industrial applications. The object of this invention is to provide a corn steeping process which can be carried out in a short period without technical difficulties.